Oven Not Reaching Temperature

Your oven says it's hot but everything takes far longer than the recipe promises. This walks the likely causes, faulty thermostat, failed element, broken sensor, a door seal leaking heat, fan failure on fan-assisted models, and tells you which a cheap oven thermometer can confirm before you pay anyone. Test before you bin it.

My oven is not reaching the temperature I set. I set it to {SET_TEMP_C} degrees C but food is taking roughly {EXTRA_TIME} longer than recipes say. It's a {OVEN_TYPE} oven and it {ANY_SYMPTOMS}. Give me a fault tree covering: faulty thermostat, failed heating element, broken temperature sensor, door seal letting heat escape, and fan failure on fan-assisted models. Tell me which I can confirm with a cheap oven thermometer and a multimeter, which parts are under a tenner, and which ones mean calling an engineer.